In a typical internal combustion engine such as a diesel engine mounted on a vehicle, decreases in the temperature of exhaust gas lowers the temperature of a bed for an exhaust gas purification catalyst, which is arranged in an exhaust gas passage. This adversely affects the purification functions of the exhaust gas purification catalyst. In such an internal combustion engine, when the exhaust gas temperature decreases, bed temperature control is executed to forcibly increase the exhaust gas temperature and correct the difference between the catalyst bed temperature and a target bed temperature. In the bed temperature control, the exhaust gas temperature is increased by performing post injection, in which a small amount of fuel is injected into the engine after the fuel injection that generates engine output, or by adding unburned fuel to the exhaust gas with a fuel addition valve, which is arranged in the exhaust gas passage. In the prior art, the apparatus described in patent document 1 is known as an exhaust gas purification controller that executes such bed temperature control.
In the exhaust gas purification controller described in patent document 1, correction amounts corresponding to injection amounts for post injection and addition amounts for unburned gas are stored as learned values for various ranges of the engine. The learned values are reflected in the present injection amount for post injection and the addition amount of unburned fuel. This obtains a stable temperature increasing effect with the post injection and the addition of the unburned fuel regardless of the operation state of the engine. As a result, accuracy and reliability for the control are increased when holding the catalyst bed temperature at the target bed temperature.
An exhaust gas purification apparatus that performs the so-called particulate matter (PM) regeneration for burning and eliminating PM by increasing the temperature of a diesel particulate filter (DPF), which is an exhaust gas catalyst, has the shortcomings described below.
Generally, in such an exhaust gas purification apparatus, the above-described learned values are calculated when certain conditions are satisfied, such as the PM regeneration being performed or the target bed temperature being 600 degrees or greater. A learned value is obtained by calculating the difference between the actually supplied fuel amount, which is the fuel amount supplied to the DPF from the addition valve, and an estimated supplied fuel amount, which is the fuel amount that actually contributes to the increase of the catalyst bed temperature increase. The actually supplied fuel amount and the estimated supplied fuel amount are calculated from the target bed temperature, an estimated exhaust gas temperature, which is the estimated temperature of the exhaust gas flowing through the exhaust gas passage, an estimated bed temperature, which is the estimated temperature of the DPF, and the amount of exhaust gas flowing through the exhaust gas passage. Among these factors, it is important that the estimated bed temperature and estimated exhaust gas temperature be calculated with high accuracy to learn the amount of unburned fuel added by the fuel addition valve. However, when the exhaust gas temperature and exhaust gas flow amount vary greatly, that is, when the exhaust gas temperature and exhaust gas flow amount are in a transitional state, a large difference is produced in the distribution of the exhaust gas temperature and the distribution of the catalyst bed temperature. In such a transitional state, it is difficult to form models with the exhaust gas temperature and the catalyst bed temperature. This drastically lowers the calculation accuracy of the estimated exhaust gas temperature and the estimated bed temperature. Thus, under such a situation, it is difficult to continue appropriate bed temperature control.
Such a problem is not limited to an exhaust gas purification controller that executes bed temperature control for the PM regeneration. The problem occurs regardless of whether the engine is a diesel engine or a gasoline engine and occurs in the same manner in an exhaust gas purification controller that executes control for increasing the temperature of a catalyst by adding unburned fuel to eliminate, for example, sulfur from the catalyst. Such a problem is not limited to an exhaust gas purification controller that mainly performs the addition of unburned fuel to the exhaust gas with the fuel addition valve and also occurs, for example, in an exhaust gas purification controller that controls the bed temperature of an exhaust gas catalyst based on the fuel additionally supplied to the internal combustion engine such as an exhaust gas purification controller that mainly injects fuel into the combustion chamber through post injection.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-172185